Introduction to Industrial Automation

Figure 7.52  Operation of a machine (C) with two time constants, T1 and T2 for OFF and ON 

statuses, correspondingly. (a) Conventional circuit, (b) time response (ON–OFF) of relay d and 

(c) implementation with PLC.

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Basic Programming Principles of PLCs 

◾

 

321

In the description of the counting instructions of Section 7.4.4, the behavior of a counter’s 

output with respect to its content was explained. The instruction “AN C10”, from which the acti-

vation of output Q2.3 is dependent, is based exactly on the condition that the counter’s output is 

logic “1” if its content is C10≠0.

In the previous example, the presetting of the counter to the value 100 was performed manu-

ally (by pressing the set button). This means that in order to operate the installation continuously, 

someone should press the button set each time that 100 luggage have passed. Subsequently, two 

different ways for counter presetting at a value will be considered. The first way will automati-

cally achieve the resetting of the counter to its initial value, while the second one will be indirect 

through a data block. More specifically, automatic resetting of the counter means that when the 

counter of the luggage in the previous example has counted 100 objects and therefore its content is 

zero, then it will obtain the value of 100 again to perform the subsequent counting. The Boolean 

language program, where the automatic resetting is achieved, has a small number of additional 

instructions in relation to the previous one (manual presetting), and also does not use the input 

I0.0 for set. The instructions of the program are as follows:

A

I0.5

CD

C10

AN M20.0

L

‘100’

S

C10

A

C10

=

M20.0

A

I0.3

R

C10

R

Q2.3

AN C10

S

Q2.3

BE

Automatic value resetting of counter

•The output activation is latched (set) since the

  content of the counter does not remain zero.  

•The relay C

1

 is deactivated by the same button

  “Reset” which nulls the counter.   

A

I0.5

CD

C10

AN M20.0

L

‘100’

S

C10

A

C10

=

M20.0

A

I0.3

R

C10

R

Q2.3

AN C10

S

Q2.3

BE

PLC

Inputs

Outputs

0 V

+24 V DC

0 V

+24 V DC

I0.0

I0.3

I0.5

Q2.3

Set

C

1

PC

Reset

PC

(a)

(b)